The biggest drugs scandal to hit sport was looming last night, with scores of the world's leading athletes facing lengthy bans for use of a previously unknown steroid.

The biggest drugs scandal to hit sport was looming last night, with scores of the world's leading athletes facing lengthy bans for use of a previously unknown steroid.

Testing will begin in Britain within the next few days on more than 1,000 recently stored samples. The tests are for THG, a performance-enhancing steroid "tweaked" by scientists in the United States to evade normal doping checks. A test specially designed to detect THG has only recently been developed, and the decision to recheck samples so comprehensively is unprecedented.

There are severe doubts over the future of Britain's number one sprinter Dwain Chambers, the fastest man in Europe and a serious contender for Olympic gold. Chambers admitted he had tested positive for THG, or tetrahydro-gestrinone. But he denied knowingly taking it.

The developments have set a timebomb for athletics authorities throughout the world, and could severely damage the future of the sport.

Bans for a number of other athletes, mainly American, could seriously threaten the quality of the 2004 Olympics in Athens. It has been reported that about 20 US athletes, including Olympic and world record holders have tested positive from samples taken at the US championships in June.

On Friday, the International Association of Athletics Federations announced all 400 urine samples it took at the World Athletics Championships in Paris in August would be unfrozen and retested for THG. National sporting bodies in Australia, Greece and Germany have also announced they would be re-examining recent batches of samples.

Michele Verroken, director of UK Sport's anti-doping unit, said that, in addition to the start of tests on the 1,000 samples stored at its Chelsea laboratory, action would be taken within the next couple of days to test any athletes known to have been training or competing in the United States.

David Moorcroft, chief executive of UK Athletics, stressed Chambers had not yet been suspended, saying the IAAF had the responsibility to see the process through.

He added that the latest process of investigating so-called "designer" steroids was "depressing", but said it was a price worth paying if the sport was to take dope-testing seriously.

Asked to compare the crisis with the previous spate of positive tests for the steroid nandrolone in the late Nineties by British athletes such as Doug Walker, Linford Christie and Mark Richardson, Mr Moorcroft said: "My suspicion is that this isn't about contaminated supplements, it's about the real McCoy cheating."

The discovery of THG was prompted this summer when an anonymous coach claimed that a number of top athletes were taking the drug.